Stable isotope and petrologic evidence for open-system degassing during the climactic and pre-climactic eruptions of Mt. Mazama, Crater Lake, Oregon

Abstract

Evaluation of the extent of volatile element recycling in convergent margin volcanism requires delineating likely source(s) of magmatic volatiles through stable isotopic characterization of sulfur, hydrogen and oxygen in erupted tephra with appropriate assessment of modification by degassing. The climactic eruption of Mt. Mazama ejected approximately 50 km 3 of rhyodacitic magma into the atmosphere and resulted in formation of a 10-km diameter caldera now occupied by Crater Lake, Oregon (lat. 43°N, long. 122°W). Isotopic compositions of whole-rocks, matrix glasses and minerals from Mt. Mazama climactic, pre-climactic and postcaldera tephra were determined to identify the likely source(s) of H 2O and S. Integration of stable isotopic data with petrologic data from melt inclusions has allowed for estimation of pre-eruptive dissolved volatile concentrations and placed constraints on the extent, conditions and style of degassing. Sulfur isotope analyses of climactic rhyodacitic whole rocks yield δ 34S values of 2.8–14.8‰ with corresponding matrix glass values of 2.4–13.2‰. δ 34S tends to increase with stratigraphic height through climactic eruptive units, consistent with open-system degassing. Dissolved sulfur concentrations in melt inclusions (MIs) from pre-climactic and climactic rhyodacitic pumices varies from 80 to 330 ppm, with highest concentrations in inclusions with 4.8–5.2 wt% H 2O (by FTIR). Up to 50% of the initial S may have been lost through pre-eruptive degassing at depths of 4–5 km. Ion microprobe analyses of pyrrhotite in climactic rhyodacitic tephra and andesitic scoria indicate a range in δ 34S from −0.4‰ to 5.8‰ and from −0.1‰ to 3.5‰, respectively. Initial δ 34S values of rhyodacitic and andesitic magmas were likely near the mantle value of 0‰. Hydrogen isotope (δD) and total H 2O analyses of rhyodacitic obsidian (and vitrophyre) from the climactic fall deposit yielded values οf −103 to −53‰ and 0.23–1.74 wt%, respectively. Values of δD and wt% H 2O of obsidian decrease towards the top of the fall deposit. Samples with depleted δD, and mantle δ 18O values, have elevated δ 34S values consistent with open-system degassing. These results imply that more mantle-derived sulfur is degassed to the Earth’s atmosphere/hydrosphere through convergent margin volcanism than previously attributed. Magmatic degassing can modify initial isotopic compositions of sulfur by >14‰ (to δ 34S values of 14‰ or more here) and hydrogen isotopic compositions by 90‰ (to δD values of −127‰ in this case).